A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In device manufacturing methods using lithographic apparatus, an important factor is the yield, i.e., the percentage of correctly manufactured devices. One factor affecting yield is the accuracy within which layers are printed in relation to layers that have previously been formed. This is known as overlay and the overlay error budget will often be 10 nm or less. To achieve such accuracy, the substrate must be aligned to the reticle pattern to be transferred with great accuracy.
A number of sensors are used at substrate level for evaluating and optimizing imaging performance. These may include transmission image sensors (TIS). A TIS is a sensor that is used to measure at substrate level the position of a projected aerial image of a mark pattern at mask (reticle) level. The projected image at substrate level may be a line pattern with a line width comparable to the wavelength of the exposure radiation. The TIS measures the aforementioned mark pattern using a transmission pattern with a photocell, i.e., a radiation detector, underneath it. The sensor data may be used to measure the position of the reticle with respect to the substrate table in six degrees of freedom, i.e., three degrees of freedom related to translation and three degrees of freedom related to rotation. Moreover, magnification and scaling of the projected mark pattern may be measured.
At wavelengths between about 10-200 nm, the radiation sensitive surface of the radiation detector of a conventional TIS deteriorates within a limited time frame. As a result, the TIS has a limited lifetime. With the continual desire to image ever smaller patterns to create device with higher component densities, there is pressure to reduce the wavelengths used. In order to maintain or reduce overlay errors, there exists a need for a more robust TIS.